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Subject: Re: [PATCH] /dev/zero: also implement ->read
To:     David Laight <David.Laight@ACULAB.COM>,
        Pavel Machek <pavel@denx.de>
Cc:     Christoph Hellwig <hch@lst.de>, "arnd@arndb.de" <arnd@arndb.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <7ece832b-e2b7-04af-f4bb-e84c909ee332@csgroup.eu>
Date:   Mon, 7 Sep 2020 06:44:24 +0200
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Le 06/09/2020 à 22:52, David Laight a écrit :
> From: Christophe Leroy
>> Sent: 06 September 2020 19:36
>> Hi,
>>
>> Le 06/09/2020 à 20:21, Pavel Machek a écrit :
>>> Hi!
>>>
>>>>>> Christophe reported a major speedup due to avoiding the iov_iter
>>>>>> overhead, so just add this trivial function.  Note that /dev/zero
>>>>>> already implements both an iter and non-iter writes so this just
>>>>>> makes it more symmetric.
>>>>>>
>>>>>> Christophe Leroy <christophe.leroy@csgroup.eu>
>>>>>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>>>>>
>>>>> Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>>>>
>>>> Any idea what has happened to make the 'iter' version so bad?
>>>
>>> Exactly. Also it would be nice to note how the speedup was measured
>>> and what the speedup is.
>>>
>>
>> Was measured on an 8xx powerpc running at 132MHz with:
>>
>> 	dd if=/dev/zero of=/dev/null count=1M
>>
>> With the patch, dd displays a throughput of 113.5MB/s
>> Without the patch it is 99.9MB/s
> 
> That in itself isn't a problem.
> What was the throughput before any of these patches?
> 
> I just remember another thread about the same test running
> a lot slower after one of the related changes.

> While this speeds up read /dev/zero (which is uncommon)
> if this is needed to get near the old performance then
> the changes to the 'iter' code will affect real workloads.

If you are talking about the tests around the set_fs series from 
Christoph, I identified that the degradation was linked to 
CONFIG_STACKPROTECTOR_STRONG being selected by default, which provides 
unreliable results from one patch to another, GCC doing some strange 
things linked to unrelated changes.

With CONFIG_STACKPROTECTOR set to N, I get stable performance and no 
degradation with any of the patches of the set_fs series.

Christophe